Why can't New Horizons send data and do observations at the same time?

The Voyager probes have a scan platform, which allowed them to point their cameras independently of the orientation of the spacecraft. New Horizons doesn't have a scan platform, so the entire spacecraft must be rotated to point the cameras in the right direction. As a result, the mission planners had to choose between observations and downlinking data at several points during the Pluto flyby.

Why wasn't a scan platform included in New Horizons?

PS: The reason I've asked this question: a couple of recent questions here are about Pluto encounter planning. The answers usually refer to NH's lack of a scan platform, but we haven't tackled the underlying reason yet.

• Whatever the reasons, I see no problem halting transmissions for some hours during observations knowing it will take many months to download that data. – Juancho Jul 15 '15 at 12:13

Mass, reliability and power. Adding the ability to rotate a device adds to the mass and power requirements of the system. It also gives more points that could go wrong. New Horizons is one of the spacecraft that has taken the longest to reach its intended target. Moving parts would have made it more complex.

Remember, the New Horizons only weights 478 kg. To add more mass would be very difficult. Hard drive space is much cheaper and better now than it was when Voyager was made. With Voyager, it was more important to send data quickly than it is with New Horizons. Voyager was also doing quite a bit longer encounter than New Horizons, due to the larger systems it visited. As a result, it was easier to store it, allowing for more science to be accomplished.

• Hard drives also have delicate moving parts. Storing the data on hard drives seems like it would add a significant risk to the mission. – kasperd Jul 15 '15 at 14:21
• New Horizons is using solid state memory. en.wikipedia.org/wiki/… and Voyager has digital tape storage en.wikipedia.org/wiki/Voyager_1#Communication_system – duzzy Jul 15 '15 at 14:34
• New Horizons is the spacecraft that has taken the longest in history to reach it's intended target - Rosetta took more than 10 years to reach 67P. +1 still – TonioElGringo Jul 15 '15 at 15:06
• Forgot Rosetta, but at least it had several flybys on the way... – PearsonArtPhoto Jul 15 '15 at 15:19
• Also note that electronics sent into space must be hardened against radiation; off-the-shelf stuff is often susceptible. en.wikipedia.org/wiki/Radiation_hardening – user151841 Jul 15 '15 at 16:39

There were several reasons for that decision:

1. Cost. According to Alan Stern, New Horizons' Principal Investigator:

Voyager also had a scan platform that allowed its remote sensing instruments to be trained on their targets even while the spacecraft was transmitting data to Earth- a high-cost technology that the New Horizons team chose to forego.

2. Reliability. The Voyagers used a scan platform. It's difficult to make sure mechanical components continue to work reliably in space. Lubricants evaporate in the vacuum, temperature swings, cold welding.

shortly after concluding operations at Saturn, Voyager 2’s science scan platform seized, and its gear and shaft were damaged because of the apparent migration of lubricant away from the gear-shaft interface due to overuse in a short amount of time.

Chris Hersman, Mission systems engineer for NH:

"We chose to avoid mechanisms, because mechanisms have a tendency to fail" (source: 'Mission Pluto', National Geographic Channel)

Secondary source for reliability as a driver for this design:

New Horizons’ elimination of excess mechanisms was implemented to save weight, shorten the schedule, and improve reliability to achieve a lifetime of at least 15 years.

The Voyager incident also provided proof that operation without a scan platform was possible:

Between Voyager 2’s encounter with Saturn in 1981 and Uranus in 1986, controllers developed a technique called “image motion compensation.” This involved moving the scan platform at slow rates, which was found to be possible despite the damage, in conjunction with thruster firings to rotate the entire spacecraft at a rate that would allow a target to be tracked long enough for imaging.